The present invention relates to a method for transforming plastic preforms into plastic bottles and to an apparatus for transforming plastic preforms into plastic bottles.
The starting point here is a method for transforming plastic preforms into plastic bottles, wherein the plastic preforms are introduced into blowing stations, wherein each blowing station comprises at least one blow mould and a stretching rod for stretching the plastic preform along a longitudinal axis of the plastic preform within the blow mould, and the plastic preforms are expanded to form the plastic bottles by being acted upon by a gaseous medium in the blow mould.
Such methods are already well known from the prior art and can be subsumed under the conventional blow-moulding processes for producing plastic bottles.
However, the inventors have recognized the need that often very particular individual material requirements are placed on plastic bottles and/or haptically perceptible requirements are placed on the individual shape of the respective plastic bottles.
In terms of requirements on the customer side, this often concerns not only, for example, a size or shape of the respective plastic bottles, but alternatively or additionally a wall thickness distribution in a longitudinal direction of the respective plastic bottles is also critical in order, on the one hand, to be able to ensure in a particularly simple manner a stability of the plastic bottle, particularly in the filled state thereof, and on the other hand to be able to adjust the weight of the plastic bottle while at the same time having mechanically stable properties.
According to knowledge available hitherto from the prior art, a wall thickness distribution in stretch-blow-moulded containers can be adjusted by a complicated interplay of a plurality of process-related influencing variables, such as pre-blowing timing, pre-blowing pressure and pre-blowing flow rate and also the heat settings in the different heating zones during the production process.
By way of example, the applicant's document DE 101 16 665 A1 discloses a method for controlling a blowing operation during the production of containers, in which a wall thickness of a container is detected immediately after production thereof and is compared with a target value of this at least one detected property, and a temperature profile generated for heating of the preforms and/or at least one blowing parameter is varied as a function of the magnitude of the comparison result in the direction of lowering this magnitude. Therefore, “wall thickness distribution” denotes the progression of a wall thickness of the plastic bottles along the longitudinal direction of the plastic bottle.
In other words, said document discloses adjusting a wall thickness distribution of the container only after the actual production of the container, and is moreover linked in particular to the heating process. During the blowing process, therefore, no adjustment of the wall thickness, i.e. adjustment of a wall thickness distribution, can take place according to said document.
In contrast, document EP 2 247 492 B1 discloses a stretch-blow-moulding method in which a wall thickness distribution can be monitored during the blowing operation within the blow mould, but said document merely shows the person skilled in the art that a wall thickness distribution can be adjusted by means of a plurality of different control parameters, such as, for example, the preform temperature, the temperature distribution in the preform, the blowing pressure curve, stretching curve, material properties, etc., wherein said document does not indicate a possibility of influencing individual blowing stations individually in respect of the wall thickness distribution of individual finished bottles.
The inventors have now recognized that controlling the plurality of complex interactions between the individual control variables is very susceptible to faults during the blowing operation and moreover requires a technically complex and cost-intensive blowing apparatus.